Resin compositions prepared by reinforcing polyamide resin with inorganic fillers such as glass fiber, carbon fiber, talc and clay are widely known. However, in the case where these reinforcing materials constituted with inorganic fillers are used, the mechanical properties and the heat resistance are improved unfortunately only when these reinforcing materials are mixed in large amounts, and the obtained resin compositions are disadvantageously increased in mass because these reinforcing materials are high in specific gravity.
Recently, there has been investigated use of cellulose as reinforcing materials for resin materials. Examples of cellulose include celluloses obtained from wood, celluloses obtained from non-wood resources such as rice straw, cotton, kenaf, bagasse, abaca and hemp, and bacterial celluloses produced by microorganisms. These are found in very large quantities on the earth. Cellulose is excellent in mechanical properties, and accordingly, the inclusion of cellulose in a resin is expected to result in an effect to improve the properties of the resin composition. Moreover, cellulose is smaller in specific gravity than inorganic fillers, and accordingly a resin composition obtained by including cellulose in a resin is fortunately free from the increase of the mass.
As the method for including cellulose in a thermoplastic resin, it is a common method in which the resin and the cellulose are mixed with each other when the resin is in a molten condition. However, in this method, cellulose is mixed in the resin while the aggregated condition of the cellulose is being maintained, and hence a resin composition in which the cellulose is uniformly dispersed cannot be obtained. Consequently, the properties of the resin composition cannot be sufficiently improved.
As a technique for improving the dispersibility of cellulose in a resin, for example, WO2011/126038 discloses that a polyamide resin composition including cellulose uniformly dispersed therein is obtained by mixing an aqueous dispersion of the cellulose fiber having the average fiber diameter of 10 μm or less and the monomer constituting the polyamide resin and by subjecting the resulting mixture to a melt polymerization. JP2013-79334A discloses that a polyamide resin composition having a good color tone is obtained by not using any acid catalyst during polymerization.